1. Field of the Invention
The present invention relates to a semiconductor device having at least two or more laminated semiconductor layers, e.g., a lamination type photovoltaic device having at least two or more power generation function units.
2. Related Background Art
Photovoltaic devices are devices for converting an incident light energy into an electric energy. Among them, a solar cell serves to convert solar light into an electric energy, and it is required for the solar cell to efficiently convert light having a broad wavelength range. For this reason, in order to attain high conversion efficiency, it is necessary to efficiently absorb light over the whole broad wavelength range. As one of the means for solving this problem, a lamination type photovoltaic device is well known in which photovoltaic devices including semiconductor layers with different band gaps as optical active layers are laminated. In this lamination type photovoltaic device, a photovoltaic device using a semiconductor material having a relatively wide band gap is arranged on the light incidence side in order to absorb light of short wavelengths having a large energy, and a photovoltaic device using a semiconductor material having a relatively narrow band gap is arranged so as to underlie the first photovoltaic device in order to absorb light of long wavelengths having a low energy and having transmitted through the overlying device, whereby light is efficiently absorbed and utilized over a broad wavelength range.
Here, an important point is that it is necessary to introduce light of wavelength ranges suitable for the individual photovoltaic devices into the devices. The reason is that in the individual photovoltaic devices, a utilizable wavelength region of incident light is restricted by the band gap of the semiconductor material used for the optical active layer. That is to say, photons each having an energy lower than the band gap of the semiconductor material are not absorbed by the semiconductor material and are unable to be utilized. On the other hand, photons each having an energy higher than the band gap of the semiconductor material are absorbed. However, since the potential energies of the electrons which can be given when exciting the electrons by the photons are limited by a width (band gap energy) of the band gap, it is impossible to utilize any of differences between the band gap energy and the photon energies. That is to say, for the lamination type photovoltaic device, it is important that only light of a short wavelength region is made incident to the photovoltaic device arranged on the light incident side, while only light of a long wavelength region is made incident to the photovoltaic device underlying that device.
As one of the means for solving this problem, a method is known in which a transparent electroconductive layer is provided as a selective reflection layer between upper and lower photovoltaic devices to be used as a reflection layer. For example, Japanese Patent Application Laid-Open No. 63-77167 discloses a method in which a transparent electroconductive layer for reflecting light of short wavelengths but transmitting light of long wavelengths is provided between photovoltaic devices. In addition, Japanese Patent Application Laid-Open No. 2-237172 discloses a method in which a thickness of the transparent electroconductive layer is adjusted so that a peak of reflectivity thereof corresponds to a maximum wavelength of spectral sensitivity of a photovoltaic device arranged on the light incidence side to increase a current value of the photovoltaic device on the light incidence side. Each of those methods aims at selectively reflecting light of short wavelengths expected to be absorbed by the photovoltaic device on the incident light side to the photovoltaic device on the incident light side by the selective reflective layer to thereby utilize light more efficiently and to enhance the conversion efficiency.
A function of connecting plural devices in series to one another as well as a function of selectively reflecting light is required for that transparent electroconductive layer. In this connection, since the transparent electroconductive later concerned acts as an external resistance in terms of an equivalent circuit, the magnitude of a resistance value directly leads to a decrease in curvature factor. From this respect, high electric conductivity is required for a material of a transparent electroconductive layer.
In general, a transparent electroconductive layer is formed by utilizing a sputtering method. As for a method, two kinds of methods are proposed: a method in which sputtering is carried out in the ambient atmosphere of Ar gas with oxide such as In2O3—SnO2 or ZnO as a target; and a reactive sputtering method in which alloy such as In—Sn or Zn is sputtered in the ambient atmosphere of mixed gas of Ar and O2. Then, according to the former sputtering method, a film having a low electric resistance and high transmittance can be formed through the sputtering process.
On the other hand, the latter reactive sputtering method has a superior advantage in that a target material can be saved, and production stop time accompanying target exchange can be greatly reduced. Thus, the latter method is suitable for the mass production.
In the lamination type photovoltaic device obtained in such a manner, light is efficiently absorbed over the whole wavelength region of the incident light. Thus, the energy of the incident light can be utilized as much as possible, and hence the high conversion efficiency can be obtained.
However, in the photovoltaic device having a large area such as a solar cell, short-circuit in defective areas of the device becomes a serious problem due to its large area. As an effective means for solving this problem, for example, it is disclosed in Japanese Patent Application Laid-Open No. 6-21493 that defective areas are separated in terms of a circuit by utilizing the fact that a current is very easy to be caused to flow through the short-circuited defective areas as compared with any of normal portions to thereby prevent reduction in conversion efficiency. For example, a processing is given in which a short-circuited portion in a defective area which is already present after formation of a film is removed, or a resistance of a member in the vicinity of the short-circuited portion is increased to passivate any of defective areas, whereby short-circuit currents are prevented from being caused to flow through the defective areas to thereby restrain reduction in conversion efficiency. This processing will hereinafter be referred to as a shunt passivation.
However, in the lamination type photovoltaic device having the transparent electroconductive layer adopted as the selective reflection layer, a problem arises in that the function of restraining reduction of conversion efficiency by the shunt passivation does not function properly. As shown in FIG. 1A, a first transparent electroconductive layer 103 and a second transparent electroconductive layer 105 are formed so as to overlie a defective area 107 in a first semiconductor layer 102 on a substrate 101 and a defective area 106 in a second semiconductor layer 104, respectively. Since the first transparent electroconductive layer 103 and the second transparent electroconductive layer 105 each have a low resistance and define a planar conduction path, short-circuit currents are caused to flow through the defective areas 106 and 107 of the first semiconductor layer 102 and the second semiconductor layer 104, respectively, to cause reduction of the electromotive force. Hence, when the above-mentioned shunt passivation is carried out (refer to FIG. 1B), a defect in the second semiconductor layer 104 is passivated, but a defect in the first semiconductor layer is not passivated at all. If the operation for generation of an electric power is carried out under this state, as shown in FIG. 2, short-circuit currents caused to flow through a defective region 207 of a first semiconductor layer 202 are two-dimensionally collected to cause reduction of the electromotive force in a normal portion of the first semiconductor layer 202. Thus, this phenomenon causes reduction in conductive efficiency of the whole device.
Even if a transparent electroconductive layer is adopted as a selective reflection layer in order to increase a photoelectric current in such a manner, since any of the defects in the first semiconductor layer is not passivated through the shunt passivation process, a substrate and the first transparent electroconductive layer are short-circuited, and hence the property of the semiconductor layer is deteriorated all the more. As a result, the essential function of the selective reflection layer is not necessarily exhibited fully so that enhancement of the conversion efficiency is insufficient. In particular, when the number of defects present inside the first semiconductor layer is large, the provision of the selective reflection layer may rather cause deterioration of the property of the lamination type photovoltaic device in some cases.